resource_auditor: skip ftp.gnu.org audit

[cho-m]

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These are either slow or rate limited as frequently hit failures in CI. Also upstream requested we minimize load on ftp.gnu.org so should just stop curl-ing mirror in our audit checks (we run these on every runner so at least 6 times per PR in a short time period)

We do something similar for homepage audit but that still runs on self-hosted runners:

brew/Library/Homebrew/formula_auditor.rb

Lines 552 to 555 in f9553b6

	# Skip gnu.org and nongnu.org audit on GitHub runners
	# See issue: https://github.com/Homebrew/homebrew-core/issues/206757
	github_runner = ENV.fetch("GITHUB_ACTIONS", nil) && !ENV["GITHUB_ACTIONS_HOMEBREW_SELF_HOSTED"]
	return if homepage.match?(%r{^https?://www\.(?:non)?gnu\.org/.+}) && github_runner

[Copilot]

Pull Request Overview

This PR adds an exclusion for ftp.gnu.org URLs from URL auditing in the ResourceAuditor class to address issue #20456. The change prevents the HTTP content audit from being performed on ftp.gnu.org URLs.

• Adds a skip condition for ftp.gnu.org URLs before the HTTP content check

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[MikeMcQuaid]

Thanks!